My chickens are totally free range. They get locked up at night and that's it. Depending on the breed a 4 ft fence could be very easy to get over. That's something you'll have to test out if you want them running around. I occasionally lose a chicken to a fox but they lead a much happier life when they are able to run around and find the food that they want. I've gone to farms to purchase chickens where the hens never even get to go outdoors. They spend their whole lives in a shed with a concrete floor covered in sand or straw. They probably wish a fox would come kill them.